Fix to avoid creating new layer by mistake

Fixes #2268 (closed)

To fix this issue, a new variable hover_start_time was added which takes its initial value in __drag_update function when the colour of the separator is changed to blue. Then in dragEnd function time difference is calculated to decide if a new layer should be added. I am not sure if this feature is required for the bottom-most or top-most separator, please let me know what should be done.

@aleb @thiblahute please review!